1. Field
The invention relates to a method and a device for carrying out frequency synthesis in a distance measuring device. The invention also relates to a distance measuring device.
2. Background Information
In construction surveying or in interior construction, for instance for three-dimensional measurement of rooms, electronic distance measuring devices are often used. They have a distance measurement range of several tens of meters and are often embodied as handheld devices. Other areas of application for distance measuring devices are geodetic and industrial surveying or measurement. The fundamental principle of distance measurement with the known devices is based on the evaluation of a change over time in a parameter of the electromagnetic radiation emitted by the device and remitted by an object aimed at. To that end, the distance measuring device is equipped with an emitter for emitting an intensity-modulated radiation. In handheld devices, this is predominantly an optical radiation in the visible wavelength spectrum, to make it easier to aim at the measurement points. The optical radiation is remitted or scattered by the measurement object being aimed at and is recorded in a receiver built into the device. From the time lag of the received modulated radiation compared to the radiation emitted by the emitter, the distance from the measurement object is obtained.
In the known distance measuring devices, pin photodiodes or avalanche photodiodes are typically used as detectors, for converting the radiation remitted or scattered by the measurement object into electrical signals. Distance measuring devices whose distance determination is based on the measurement principle known as phase measurement are very commonly used. In such devices, the electrical signal received has a mixer frequency superimposed on it to produce a low-frequency measurement signal, directly at the avalanche photodiode or downstream of a preamplifier. The phase of this low-frequency signal is determined and compared with the phase of a reference signal. The difference in the measured phase of the low-frequency measurement signal and the phase of the reference signal is a standard for the distance of the measurement object.
For employing the measurement principle based on the evaluation of the phase difference, a laser modulation frequency preferably higher than 100 MHz, and a mixer frequency that differs from the laser modulation frequency only by the amount of the low frequency, are needed. It is substantially easier to determine the phase of the low-frequency measurement signal than to determine it from the original high-frequency signal received. Typically, therefore, the low frequency is selected to be in the kilohertz range, such as 10 kHz. If the laser modulation is done at a high frequency of 400 MHz, for instance, then to generate the desired low-frequency measurement signal a mixer frequency must be generated that differs from the high frequency by only 25 ppm. To that end, the highest possible crosstalk damping is required for both high frequencies. With such high frequencies located so close together, unwanted sidebands, however, can only, if at all, be supressed at extraordinarily high filtration effort and expense. Such provisions are expensive and often create further sources of error.
In most of the devices known from the prior art, the two high frequencies are generated with two separate quartz oscillators. One of the two quartz oscillators is voltage-controlled and is regulated in a phase locked loop (PLL) to the frequency that differs from the high frequency of the other quartz oscillator by the amount of the low frequency. The quartz oscillators used to generate the two high frequencies must match one another within very close tolerances. To assure the close tolerances, complicated and expensive production methods are required. Moreover, quartz oscillators can be usefully produced economically only for frequency ranges of approximately 100 MHz. For modulation frequencies greater than 100 MHz, additional frequency multipliers are needed, which entail additional costs. Alternative ways of achieving the generation of high frequencies on the far side of 100 MHz employ surface acoustic wave (SAW) resonators or filters in addition to the quartz oscillator.
Another known principle for generating the modulation high frequency and the mixer frequency differing only slight from it is direct digital synthesis (DDS). In this method, with a quartz oscillator, a frequency is generated that is multiplied electronically. From the thus-attained frequency, with the aid of a digital phase accumulator, a cosine table, and a digital/analog converter following it, the mixer frequency that differs by the low frequency is generated. To generate the actual high frequencies for the modulation frequency and the mixer frequency, further frequency multipliers are required. This variant way of generating the two high frequencies does require only a single quartz oscillator. However, the method is very complicated in terms of circuitry, and because of the many electronic components required, it has high current consumption. This is a decisive disadvantage, however, especially for relatively inexpensive handheld devices.